1. Field of the Invention
The invention relates to a low temperature process for the separation of CO.sub.2 from a gaseous mixture containing CO.sub.2 and light hydrocarbons by a multistage distillation, wherein the gaseous mixture to be fractionated is separated, in a first fractionating stage, into an overhead fraction containing essentially all of the C.sub.1 and C.sub.2 hydrocarbons, as well as a portion of the CO.sub.2, and into a bottoms fraction containing essentially C.sub.3+ hydrocarbons and the largest portion of the CO.sub.2 ; whereupon the bottoms fraction is subsequently separated into a substantially pure CO.sub.2 fraction and a C.sub.3+ hydrocarbon fraction.
2. Description Of The Prior Art
In the distillation of light hydrocarbons, especially C.sub.1 to C.sub.6 hydrocarbons having a relatively high proportion of CO.sub.2, problems are encountered involving the freezing out of the CO.sub.2. Such problems can occur, in particular, in the processing of CO.sub.2 -rich natural gases, i.e., natural gases having a CO.sub.2 content of at least about 5%, or in tertiary petroleum extraction processes wherein CO.sub.2 is injected under high pressure into deposits, i.e., underground deposits, and in addition to the recovered petroleum, an accompanying gas is obtained which contains light hydrocarbons and can include, for example, between 5 and 95% CO.sub.2. This variation is caused by the CO.sub.2 content gradually rising during the course of tertiary oil extraction, starting from a relatively low level to a very high level whereas the quantity of the light hydrocarbons contained in the gas remains essentially constant. While the CO.sub.2 is to be mainly separated as an undesired impurity in CO.sub.2 -rich natural gases it is also, in the field of tertiary petroleum extraction, a desired product stream which is to be reused by reinjecting under high pressure into the deposit.
A known procedure for the separation of CO.sub.2 from light hydrocarbons provides for a separation of a C.sub.1 fraction from the mixture, in a first fractionating stage, followed by a fractionation of the remaining C.sub.2+ -CO.sub.2 mixture into CO.sub.2 and into a C.sub.2+ fraction in a further fractionating stage. However, a number of difficulties occurs in this fractionation. While separating CH.sub.4 and CO.sub.2 under the conditions usually prevailing in demethanizing, solid CO.sub.2 deposits form in the fractionating column. During the subsequent separation of the CO.sub.2 and C.sub.2+ hydrocarbons, CO.sub.2 forms an azeotropic mixture with ethane, the azeotrope having a CO.sub.2 :C.sub.2 ratio of about 2:1, so that effective fractionation of this mixture by distillatory methods is impossible without taking additional measures. Such additional measures include, for example, the so-called RyanHolmes process (Hydrocarbon Processing, May 1982, page 131), the introduction of additives which prevent deposition of solid CO.sub.2, or are intended to break the CO.sub.2 -ethane azeotrope. Since these modes of operation are relatively energy intensive, a process requiring less energy has been proposed in assignee's U.S. application Ser. No. 743,727, filed June 12, 1985 (German Patent Application P No. 34 22 158.1); in this process, the first fractionating stage is not a demethanizer, but rather effects separation of essentially all the C.sub.1 and C.sub.2 hydrocarbons. The bottoms of this first fractionating stage yields, besides the predominant portion of CO.sub.2 contained in the gaseous mixture, the C.sub.3+ hydrocarbons which are fractionated in a single-stage or, especially in case of very high CO.sub.2 contents in the gaseous mixture, a two-stage distillation, into CO.sub.2 and a C.sub.3+ hydrocarbon fraction.